1. Field of the Invention
This invention generally relates to a fluid diffuser and method for constructing the same, and more particularly, to a fluid diffuser for use with a salt water solar pond.
2. Discussion of the Prior Art
Artificial salt water solar ponds are presently used as solar collectors in order to provide a source of low-grade heat for conversion into electricity. Such ponds have a three layer regime: an upper convective wind-mixed layer at the surface with an average salinity of 3-5 percent, and a depth of approximately 30-50 centimeters, depending upon wind conditions; an intermediate, non-convective layer termed a halocline, about 1-1.5 meters deep, with a salinity that increases uniformly with depth from about 5 percent at the top of the layer to about 30 percent at the bottom of the layer; and a lower heat-storage layer, approximately 3-5 meters deep, depending upon the amount of heat-storage desired, with a uniform salinity of about 30 percent.
Solar radiation incident on the surface is absorbed within the layers of the pond. Heat absorbed within a stratum of the wind-mixed layer reduces the density of the stratum, and creates buoyant water which quickly reaches the surface, dissipating the absorbed heat into the atmosphere. Thus, the temperature of the wind-mixed layer approximates ambient temperature. However, heat absorbed in the halocline and in the heat-storage layer is trapped in these layers because of the non-convective nature of the halocline.
As is well-known, a halocline has such a strong, downward salinity gradient that the resultant density profile also increases with depth even as the temperature also increases with depth. As a consequence, the halocline is a non-convective layer wherein heat conductivity is reduced to the molecular level. The halocline, thus, insulates the underlying heat-storage layer; and the absorption of solar radiation establishes a temperature profile in the pond which matches the salinity profile. The halocline thus serves as a transparent, insulating cover for the heat-storage layer, and protects the latter against convective heat loss to the atmosphere. From actual experience with solar ponds, the halocline is remarkably stable for long periods of time, because the rate of salt diffusion is so low.
However, the halocline is particularly sensitive to the effects of uncontrolled mixing which disrupts the downward salinity gradient of the halocline. This uncontrolled mixing destroys the ability of the halocline layer to function as an insulator by allowing hot water in the bottom zone to circulate to the surface, and thus the ability of the pond to function as a solar collector.
Uncontrolled mixing between the halocline and heat-storage layer can occur due to turbulence created in the heat-storage layer during the extraction and return of the hot brine which is typically utilized as a low temperature heat source for use with Rankine cycle turbines or the like.
In commercial solar ponds presently in operation, large, flat-plate diffusers are employed to extract and return the hot brine of the heat-storage layer. These known flat-plate diffusers include a pair of flat, stamped or machined circular plates which are equally spaced from one another so as to form a cylindrical peripheral opening that communicates with the heat-storage layer of the pond. One plate is provided with an aperture for connection to a fluid circuit external to the pond in order to extract hot brine to be delivered to a boiler supplying vaporized working fluid to a turbine, or to deliver cooled brine from the boiler. Typically, the extraction diffuser is positioned in the heat-storage layer just below the halocline where the temperature of the layer is a maximum. The return diffuser for exhausting the cooled brine after heat extraction, is positioned below the extraction diffuser at a depth where the temperature of the heat-storage layer approximates the temperature of the cooled brine.
In order to reduce turbulence and prevent unintentional mixing between the halocline and heat storage layer, conventional diffusers are designed to achieve very small flow velocities, e.g., approximately one centimeter per second at the periphery of the diffuser in a radial direction. The requirement for such low velocities results in diffusers of large size because of the large volume of brine which must be transferred from and to a boiler in a commercial salt water solar pond. For example, ponds are currently under construction where the mass flow from the heat storage layer through a diffuser may be as high as 150 m.sup.3 /hr. with an exit speed of 1 cm/sec, a diffuser 1 m high must be about 6 m in diameter.
The volumetric flow rate of known flat-plate diffusers can be increased by enlarging the overall diameter of the diffuser. However, these diffusers are bulky and prove to be an expensive solution due to the high cost of producing large diameter plates which are formed by a stamping or machining operation. It is, therefore, an object of the present invention to provide a new and improved fluid diffuser that reduces turbulences and enables the use of increased flow velocities which permits the diffuser to be smaller, and more compact in diameter, and which, above all, can be made at a reduced cost as compared to a conventional machined or stamped diffuser.